1. Technical Field
The present disclosure relates generally to radio frequency (RF) integrated circuits, and more particularly, to high gain RF power amplifiers with a negative capacitor.
2. Related Art
Generally, wireless communications involve an RF carrier signal that is variously modulated to represent data, and the modulation, transmission, receipt, and demodulation of the signal conform to a set of standards for coordination of the same. Many different mobile communication technologies or air interfaces exist, including GSM (Global System for Mobile Communications), EDGE (Enhanced Data rates for GSM Evolution), and UMTS (Universal Mobile Telecommunications System) W-CDMA (Wideband Code Division Multiple Access). More recently, 4G (fourth generation) technologies such as LTE (Long Term Evolution), which is based on the earlier GSM and UMTS standards, are being deployed. Besides these mobile communications modalities, local area data networking modalities such as Wireless LAN (WLAN)/WiFi, WiMax, and so forth.
A fundamental component of any wireless communications system is the transceiver, that is, the combined transmitter and receiver circuitry. The transceiver encodes the data to a baseband signal and modulates it with an RF carrier signal. Upon receipt, the transceiver down-converts the RF signal, demodulates the baseband signal, and decodes the data represented by the baseband signal. An antenna connected to the transmitter converts the electrical signals to electromagnetic waves, and an antenna connected to the receiver converts the electromagnetic waves back to electrical signals.
The output of the transmitter is connected to a power amplifier, which amplifies the RF signals prior to transmission via the antenna. The receiver is connected to the output of a low noise amplifier, the input of which is connected to the antenna and receives inbound RF signals. A transmit/receive switch selectively interconnects the antenna to the output of the power amplifier during transmission, and to the input of the low noise amplifier during reception. Thus, the power amplifier, the low noise amplifier, and the antenna switch serves as key building blocks in RF transceiver circuitry. These components may be referred to as a front end circuit.
In most wireless communications such as the aforementioned WCDMA and 3G/4G cellular, it is important for the RF power amplifier to have high efficiency. Typically, a power amplifier is comprised of several stages, each of which contributes to define an overall gain from the transceiver to the antenna. There are various factors that limit the gain of the RF power amplifier, including those that relate to implementation as integrated circuits. One limitation is the source or emitter degeneration inductance resulting from connecting the source terminal or the emitter terminal of the power amplifier transistor to package ground. This connection is made via bond wires, flip-chip balls, or through-silicon vias (TSV) in various implementations.
The inductance associated with this circuit segment is understood to substantially reduce gain. Even a modest 0.35 nH inductance, which is typical of a short bond wire, can result in almost 8 dB reduction in gain. Multiple parallel bond wires may be utilized to counteract this and increase gain, but may not be practical because of limited die area and sensitivity. The alternative of TSVs have a higher cost and susceptible to degradation attributable to mechanical stress. Although flip-chip wafers are well suited for modular applications, the compatibility of devices based thereon with conventional chip-on-board (COB) applications remain problematic.
Prior solutions have contemplated the addition of a resonance circuit (inductor and capacitor) connected to the source or emitter and tuned for series resonance at the operating frequency. This is understood to reduce the influence of the degeneration inductance and thereby increase the power amplifier gain. However, this configuration is understood to lead to stability issues at the power amplifier stage.
Accordingly, there is a need in the art for low cost circuit solutions with increased gain in RF transistor stages. There is a need for high gain RF power amplifiers with a negative capacitor.